With as much meaning as a silly, scatological movie can muster, “Step Brothers” mocks the measurement of male maturity in a world where many “kids” have moved back in with parents.

Yes, the path of a modern-day man-child is a well-trodden trail for ubiquitous producer Judd Apatow (“Knocked Up,” “The 40-Year-Old Virgin”), and “Step Brothers” feels like the exhaustion point of that idea. (It helps that it’s short, not pushing Apatow’s usual 120-minute mark.)

Yet “Brothers” extends, to feature length, the airy viral-video jabs on Ferrell and McKay’s Funny Or Die Web site, with surreal absurdity and goofy comic commitment from Ferrell and co-star John C. Reilly (who also co-created the storyline). The pair, who made a decent comedy team in “Talladega,” truly soars when not tethered to slavish NASCAR tie-ins.

Ferrell and Reilly are Brennan and Dale, unemployed Californian 40-somethings living with their single parents. Their developments aren’t arrested. They’re serving life sentences in Chino.

Breakfast and self-love are morning routine for Brennan. He’s been called the “songbird of his generation,” but he hasn’t sung a note since a traumatic “Pirates of Penzance” incident in high school triggered by his viciously overachieving younger brother, Derek (Adam Scott).

In his junior year, Dale dropped out of college to join the family business. Too bad it’s medicine. Now, he wears “movie-quality” Chewbacca masks and conducts experiments in his “beat laboratory” (a drum room) with dreams of superstardom. (If “Talladega” flipped a bird to NASCAR, “Brothers” does so to the “American Idol” anyone’s-a-star mentality, with the finger in its pocket.)

After Brennan’s mom (Mary Steenburgen) and Dale’s dad (Richard Jenkins) meet and marry, the new step brothers clash with pouts and pranks after the families merge.
Here, the movie goes a little too long with profane shouting matches between the “boys.”

It’s the little pouting-rant details of Ferrell and Reilly’s inner 12-year-olds that are better, such as perfectly clinched near-tears faces or fishing for whispered taunts at each other in a shared bedroom. (Credit costume designer Susan Matheson and production designer Clayton Hartley for making this setting, and their wardrobe, a perfect petri dish of retro pop culture.)

Eventually bonding over a shared “favorite non-porno mag to masturbate to,” Brennan and Dale are soon enjoying karate together in the garage and Steven Seagal movies in the den. It’s when they’re forced to find work, and, eventually, fix a fast-fracturing family, that the film finds footing.

That’s because “Step Brothers,” in due time, doesn’t easily cast aside family members who have sour feelings as vile villains. It’s nowhere near an integral blend of comedy and drama as in “Stuart Saves His Family,” but it works. (One man calling another “Dragon” is a strangely moving gesture.)